Resonant and semi-resonant DC-DC voltage converters, including isolated and non-isolated topologies, are used in a variety of applications including telecommunications, consumer electronics, computer power supplies, etc. The usage of such converters is gaining popularity because of their zero-voltage and/or zero-current switching characteristics, and their ability to utilize parasitic electrical properties inherent in an electronic circuit. Such converters provide advantages including lower cost and higher efficiency as compared to other types of converters.
Many resonant and semi-resonant voltage converters include a power stage comprised of a high-side power switch and a low-side power switch. Furthermore, such converters include another low-side power switch, herein termed a synchronous rectification (SR) switch, through which a current taking the shape of the upper half cycle of a sinusoid flows when the SR switch is enabled (conducting). In order to achieve optimal efficiency, the high-side switch in the power stage should be switched when the voltage across it is zero (zero-voltage switching), whereas the SR switch should be switched when the current through it is zero (zero-current switching). In order to meet load requirements which may vary over time, such voltage converters are often controlled to switch the power switches using a variable switching frequency. Hence, the switching periods for the power switches cannot be fixed. Additionally, the circuit resonance leading to the half-cycle sinusoidally shaped current is caused by a reactance which can vary according to environmental factors, e.g., temperature. Accordingly, the most practical means for achieving zero-current switching through the SR switch is to estimate the current flowing through it on a cycle-by-cycle basis, so that an instant in time when this current crosses zero may be accurately detected for each cycle and the SR switch can be disabled at such an instant in time.
Prior techniques for estimating the SR switch current, in order to detect a time instant when this current crosses zero, are based upon measuring the voltage across the SR switch and using the drain-to-source resistance of the switch (Rdson) to derive an estimated SR current. Such techniques have the disadvantages that the drain-to-source resistance Rdson has a temperature dependency leading to inaccurate current estimates, extra circuitry may be required for blocking a common mode voltage, and noise and body diode conduction in the SR switch may lead to poor accuracy. The net effect of these current estimate inaccuracies is that the instant in time when the SR switch current crosses zero is not accurately detected which, in turn, leads to the SR switch being turned off when there is still a not insignificant amount of current flowing through the SR switch.
Accordingly, there is a need for improved techniques for estimating the current flowing through the SR switch in a voltage converter, so that the zero-crossing point may be accurately detected and the SR switch may be turned off at a near-optimal time. These techniques should provide an accurate current estimation and should require minimal additional circuitry. Furthermore, techniques should be provided for sensing the current in portions of the voltage converter other than the SR switch itself, so as to allow for flexibility in the implementation of the voltage converter in different applications, while also providing for accurate zero-cross detection of the SR current.